Let`s start with the facts we are sure of: this plant resembles the Lemon Verbena. It can be used for infusions and it got a fragrance like spearmint chewing gum. It is a fast grower and leaves can be harvested in large quantities. It isn`t hardy at all and got to be kept on a sunny and warm window sill during winter. 

The rest is quite mysterious. Already the scientific name is dubious, for it appears nowhere in the botanical books and floras we looked up. Besides there is a mistake in `polystacha`, it should read `polystachia` or `polystachya`. Reportedly it is native to Argentina where it is said to be called `Burro` (= `Donkey`).  Strange to say this name is used only by Dutch and German nurseries. Most of these nurseries provide the information that `Burro` was used to flavour Paraquai Tea (`Mate`), but this use seems to be unknown elsewhere. No matter which information we tried to verify, we always were unsuccessfull and the origins of the rumour were hidden in fog. We guess name and story of this plant are faked up, probably by a nursery in a German speaking country. Most likely it is a member of Verbenacea, but we can`t tell if it belongs to Lippia, Aloysia or the any other genus.